The subject matter of the present invention pertains to means for controlling the deflection of an electron beam within a cathode-ray tube, and especially within such a tube employing multiple electron guns and a shadow mask.
The general construction of a conventional three-gun, shadow-mask type, cathode-ray tube and the manner in which it is operated to produce a raster-scan color image are well known to the art. Equally known to the art is that, absent dynamic correction, the image produced by such a tube will contain certain inherent distortions. Primary among these are pincushion distortion caused by the center of deflection of the three electron beams being located apart from the center of curvature of the tube's display screen (present in monochromatic as well as color CRT's), trapezoidal distortion caused by at least two of the electron guns being located off the longitudinal axis of the tube envelope, and misconvergence of the beams at the tube's shadow-mask caused by the guns being displaced from one another laterally. With a delta-gun configuration, all three guns are spaced about the longitudinal axis of the gun assembly; with an in-line configuration, one gun is located on axis and the other two are spaced at either side.
The usual method of correcting geometric distortion is to impress certain analog correction factors onto the deflection signals used to deflect the beam or beams back and forth across the display screen to produce the image raster. Misconvergence is usually corrected by a similar impression of different analog correction factors onto the magnetic fields used to converge the three beams at center screen. Of the two distortions, the most difficult to correct accurately and uniformly, and one which requires periodic adjustment, is that of misconvergence.
Basic schemes for accomplishing dynamic beam convergence include the production of individual vertical and horizontal signals for each of the beams within the tube. Approximating somewhat the form of slightly skewed parabolas, the correction signals provide zero correction at center screen and increasing correction as the beams are deflected away from center. Such a basic approach is usually adequate for a home television environment where viewers are not overly critical and viewing distances are on the order of 6 to 10 ft. In the field of information display, however, where viewers are more critical and viewing distances much shorter, and more importantly, where resolution requirements are much more strict, the amount of misconvergence left uncorrected by such a basic approach is unacceptable.
An improvement over the basic scheme described above is exemplified by the 4027 color graphics terminal produced by Tektronix, Inc., the assignee of the present invention, wherein the display screen is divided into several sub-areas and different correction signals, independently adjustable, are generated for each such division. Such an approach permits a more accurate convergence of the three beams over the entire area of the screen. In the 4027, the display screen is divided into nine sub-areas and the beams may be converged in each such area by the adjustment of three potentiometers, one for each beam. Although providing increased correction, such a scheme still requires the somewhat time-consuming adjustment of 27 different potentiometers, three for each of the nine sub-areas. Other known schemes divide the display screen into an even greater number of sub-areas (the Tektronix 690 color monitor, for example, uses thirteen) and require the attendant adjustment of an even greater number of potentiometers. A common disadvantage of such schemes is the requirement for a human operator to assume full control of the display system for the time necessary to perform the several adjustments at each individual sub-area.
More recent developments include digital convergence schemes wherein correction information may be entered digitally, via a keyboard or other similar means, for conversion into analog signals producing the desired amount of beam adjustment. Examples of such schemes includes those disclosed by Hallett et al. U.S. Pat. No. 4,203,051 and its companion Sowter U.S. Pat. No. 4,203,054, both of which are assigned to IBM, and the SRL Model 382 color display developed by Systems Research Laboratories, Inc., of Dayton, Ohio. The IBM scheme is also described in an article by J. S. Beeteson et al. "Digital System for Convergence of Three-Beam High-Resolution Color Data Displays" appearing at page 598 of the September 1980 issue of IBM J. Res. Develop., Vol. 24, No. 5. A description of the SRL convergence scheme may be found in a paper entitled "A 25-In. Precision Color Display for Simulator Visual Systems" by R. E. Holmes and J. A. Mays of Systems Research Laboratories. A common characteristic of both the IBM and SRL systems is the use of a keyboard permitting operator entry of digital information representing the degree of movement necessary for each of the three beams to accomplish their convergence or other geometric adjustment. The IBM system permits the beams to be individually adjusted at 13 different points over the display area, while the SRL system permits adjustment at 256 different points.
A semi-automatic scheme for performing deflection adjustments only is disclosed in Bristow U.S. Pat. No. 4,099,092. In that scheme, a photodiode array or solid-state imaging camera positioned in front of a CRT display, and a digital computer, are employed to generate correction factors for later application, via a programmable read-only memory, to the conventional deflection waveforms.
A common disadvantage of all known prior art schemes is that a human operator is still required to assume full control of the system during the time necessary to perform the convergence or geometric correction operation.